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Tor uses another circuit to talk to the other side and this other circuit is faster than the one you used previously. If this is true, you could also have the effect that your download starts e.g. with 3 MB/s and resumes with 1 MB/s after you paused it and Tor switched the circuit (see this).

Normally, it does not compromise your anonymity if Tor switches circuits. The other side can obviously come to the conclusion that you switched Tor circuits. But what should the other side do with this information? It only knows two exit nodes that you have used (ignoring attack vectors on Tor itself).

Your current circuit just jumped to a higher speed. A simple reason is that another user that used your exit node for watching movies or downloading files or whatever that uses high data rates switched to another exit node or just closed the connection. Then, less data has to be transferred over your exit node and thus the whole circuit and therefore your download speeds up.

This is also the case when you had e.g. two downloads at the same time. If the first finished, the second one gets the "full" bandwidth speed.